1. Field of the Invention
The present invention relates to an illuminated position adjusting method in an optical scanning apparatus. The present invention is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which employs an electrophotographic process.
2. Description of the Related Art
In a color image forming apparatus using an optical scanning apparatus, plural of scanning lines are superimposed on each other to form an image, and hence particularly, it is important to reduce a misalignment of scanning lines among respective colors.
Various optical scanning apparatuses that are installed in such color image forming apparatus have conventionally been presented (see Japanese Patent Application Laid-Open No. H11-326804 and Japanese Patent Application Laid-Open No. 2005-004050).
In Japanese Patent Application Laid-Open No. H11-326804, an alignment of inclination and the curvature of scanning lines in respective optical scanning apparatuses are adjusted by displacing positions of imaging optical elements in the respective scanning apparatuses, to thereby suppress the misalignment of the scanning lines among respective colors in a sub-scanning direction.
Meanwhile, in recent years, in an optical scanning apparatus, as a way to downsize the entire apparatus, there is used a so-called opposed scanning optical system that scans a plurality of light beams by the same (single) optical deflector, and guides the light beams onto respective different surfaces to be scanned by imaging optical units which are disposed on both sides of the optical deflector.
Further, with an aim to realize a color image forming apparatus that performs color image formation of four colors (Y, M, C, and K) by using a single optical deflector, there is used a sub scanning oblique incident optical system that allows a light beam to enter a deflection surface of the optical deflector from an oblique direction within a sub scanning section.
In Japanese Patent Application Laid-Open No. 2005-004050, in the opposed scanning optical system and the sub scanning oblique incident optical system, there is provided a light blocking member that blocks an undesirable light so as to prevent the undesirable light from being allowed to enter the imaging optical element. That is, the undesirable light spatially separated from an actual ray within the sub scanning section is blocked by using the light blocking member. As a result, the undesirable light is prevented from arriving at the surface to be scanned by means of the light blocking member.
In the above-mentioned color image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H11-326804, the optical elements are eccentrically parallelized within the sub scanning section in all of the optical paths of the optical scanning apparatus, to thereby adjust the illuminated positions on the surface to be scanned. For this reason, there is an adverse effect that an imaging spot shape on the surface to be scanned is deteriorated due to the eccentric-parallelizing of the imaging optical elements within the sub scanning section.
Further, in the case of an optical scanning apparatus in which a large number of reflector mirrors are disposed as light beam reflection members within an optical path for the purpose of downsizing the optical scanning apparatus, which has been highly demanded in recent years, a deviation amount in the illuminated position increases due to an insufficient precision in assembling the reflector mirror.
With the above-mentioned action, a parallel eccentric amount within the sub scanning section of the imaging optical element for adjusting the illuminated position also increases. Therefore, the imaging spot shape on the surface to be scanned is deteriorated to the degree that a high-grade image cannot be achieved.
Further, when the illuminated position is adjusted by eccentrically rotating the reflector mirror within the sub scanning section in all of the optical paths of the optical scanning apparatus, there is required an adjusting mechanism for precisely performing the eccentrically rotating of the respective reflector mirrors. As a result, a space for providing the adjusting mechanism is required, and the entire optical scanning apparatus is increased in size.
Further, in the opposed scanning optical system in which the imaging optical systems are symmetrically disposed with the optical deflector sandwiched therebetween as disclosed in Japanese Patent Application Laid-Open No. 2005-004050, there are provided light blocking members (undesirable-light blocking members) for blocking a light beam (undesirable light) reflected by each surface of the imaging optical elements so as not to guide the light beam to the surface to be scanned.
However, when the imaging optical element is largely eccentrically parallelized within the sub scanning section for the purpose of adjusting the illuminated position, there arises a problem that the undesirable light is deviated from the optical path of the undesirable light reflected by each surface of the imaging optical elements, and the undesirable light arrives at the surface to be scanned without being blocked by the light blocking member.